The present invention relates to a prosthesis for the basal joint of the thumb.
Due to a number of factors, including age and wear, the joint between the trapezium bone of the wrist and the first metacarpal (the bone at the proximal end of the thumb) often degenerates, resulting in carpo-metacarpal arthritis. Additionally, STT (scaphoid, trapezium, trapezoid) arthritis may result at the proximal end of the trapezium. These conditions may necessitate inserting a prosthesis for the basal joint of the thumb.
Prior art prostheses, such as that disclosed by Whipple U.S. Pat. No. 5,702,469, have attempted to resolve the carpo-metacarpal arthritis problem by providing a ball and socket arrangement (See FIG. 2 of Whipple '469) wherein the metacarpal component includes a rod S1 driven into the first metacarpal, with the ball portion projecting proximally from the first metacarpal, and the trapezium component having a rod S2 driven into the trapezium, with a concave surface projecting distally from the trapezium. This concave surface receives the ball of the metacarpal component for limited rotational support of the first metacarpal on the trapezium. This prior art design relies on a ligament between the first and second metacarpals to provide support and prevent dislocation of the first metacarpal from the trapezium.
In Whipple and other prior art designs, the installation is hindered by the need to save the ligament between the first and second metacarpals in order to have this ligament provide the necessary support to prevent the dislocation of the ball from its socket. This, combined with the usage of the existing trapezium to anchor the trapezium component, results in very limited accessibility for installation of a ball and socket arrangement. The result is a ball which can easily be dislocated from the member that supports it, which causes failure of the prosthesis.